Radiotherapy is used to treat cancers and other ailments in mammalian (e.g., human and animal) tissue. In a radiation therapy treatment session, a high-energy beam is applied from an external source towards a patient to produce a collimated beam of radiation directed to a target site of a patient. The placement and dose of the radiation beam must be accurately controlled to ensure the patient receives the prescribed radiation, and the placement of the beam should be such that it minimizes damage to the surrounding healthy tissue, often called the organ(s) at risk (OARs). One way to improve the accuracy of the beam placement is by the acquisition of planning images, where one or more image is acquired with the patient in the intended treatment position. CT is often the most used primary image, but can complemented with secondary datasets such as MRI, positron emission tomography (PET), ultrasound, single photon emission computerized tomography (SPECT), or other medical imaging modalities which can be registered or fused to the primary dataset to aid with anatomical visualization.
In some cases, 4D planning images can be acquired. 4D techniques have been developed to account for respiratory motion, assuming that respiration is reproducible from cycle to cycle (which is not always the case). The respiratory cycle is sub-divided into bins, for example 10-12 of equal spacing, and an image is produced for every bin by consolidating image information over many imaging cycles. Techniques for this process have been applied to 4D-CT, 4D-MRI, 4D-PET, 4D-ultrasound, and other modalities. Although these techniques are useful for targets that are primarily influenced by respiratory motion, they do not take into account larger variations of patient anatomy over time. Nor are they relevant to organs where other motion processes dominate such as digestive processes, peristalsis, bladder filling, etc.
Physicians can use the planning images to identify and contour a target (e.g., a diseased organ or a tumor) as well as OARs. Contouring can be performed manually, semi-automatically, or automatically. A treatment contour, often referred to as a planned target volume (PTV), is created which includes the target contour plus sufficient margins to account for microscopic disease as well as treatment uncertainties. A radiation dose is prescribed, and a treatment plan is created that optimally delivers the prescribed dose to the PTV while minimizing dose to the OARs and other normal tissues. The treatment plan can be generated manually by a user, or automatically using an optimization technique.
A treatment course is developed to deliver the prescription dose over a number of fractions, each fraction delivered in a different session. For example, 30-40 fractions are typical but 5 or even 1 fraction can be used, and fractions are often delivered once or in some cases twice per weekday. In some cases, the radiation treatment plan can change throughout the course to focus more dose in some areas.
In each fraction, the patient is set up on the patient support accessory (often a “couch”) of the linear accelerator and repositioned as closely as possible to their position in the planning images. Unfortunately, this is an impossible task to carry out accurately in practice, since the patient is not a rigid object and the anatomy can move. Fraction-to-fraction motions are often referred to as interfractional motion, while motion occurring during a fraction itself is often referred to as intrafractional motion. Image guided radiotherapy (IGRT) attempts to solve the problem of interfractional motion, which is in many cases the larger of the two types of motion. As opposed to planning images, which can be acquired on any diagnostic scanner, IGRT images must be acquired directly in the treatment room, while the patient is in treatment position. Technologies for IGRT imaging that have been developed are cone-beam CT (CBCT), ultrasound, MRI, portal imaging, CT-on-rails, on-board kV imaging, and others have been either proposed or in development. In some cases, anatomical contrast is low in IGRT images, and fiducial markers are implanted in the patient to help with visibility of the target. Some technologies have been developed that do not use imaging at all, but rely on the imageless detection of the position of active fiducials, for example by implanting radiofrequency (RF) beacons. This is generally still referred to as IGRT, even though strictly speaking, images are not obtained. For generality, we will refer to ‘images’ as to include positional information of fiducials, or any data collected about the patient's interfractional state, such as target or OAR positions, rotations or deformations, blood pressure, heart rate, weight, deformation, etc.
IGRT refers to not only the collection of image information, but also how to compensate for interfractional state. IGRT images are first compared to the planning images to find changes. A full deformable change over the whole patient anatomy can be found, but it is standard to focus on global shift and/or rotations that match the images as closely as possible. For example, only the shift and rotation of the target itself can be considered, or in some cases bony anatomy or an OAR, or combinations thereof. Once the shifts, rotations and/or deformations have been calculated, the treatment plan is modified to account for these changes. In many cases a complete re-plan is not practical, so the couch is simply shifted to re-align the patient as closely as possible. In other cases, full or partial re-plans are carried out. The IGRT workflow is often used to refer to the entire process of imaging, calculating a correction, and physically carrying out the correction, prior to irradiating the patient.
Each IGRT modality has its advantages and disadvantages. For example, CBCT or stereoscopic kV x-rays are often used because they are x-ray based and thus similar in nature to planning CT images, and can be integrated directly into the linear accelerator. Depending on the target site, fiducials are often inserted into the target with these modalities to enhance visibility. 3D ultrasound has also been used for IGRT, and MRI imaging has more recently been introduced by integrating the MRI into the radiation treatment room.
IGRT compensates for interfractional motions, rather than intrafractional motions. In some cases where respiratory motion dominates, 4D phase-binned IGRT techniques can be used, such as 4D-CBCT. These techniques do not consider other components of motion, and are not applicable to organs such as the prostate, GYN, breast or head and neck where other intrafractional processes dominate. Furthermore, it is often desirable to track the target directly at each point in time during the treatment, and compensate for the tracked motions on the fly. The problem is that some image-based IGRT techniques, such as CBCT or MRI, have a finite acquisition time that is overly long to track the target sufficiently fast. For example, CBCT often takes 1 minute, and MRI imaging often takes 1-3 minutes for a full 3D scan. For this reason, real-time imaging modalities have been developed for target tracking.
We refer to the term target tracking to mean measuring changes in the patient's state quickly enough to accurately represent the motion—for example, at an interval smaller than the respiratory cycle if the target undergoes respiratory motion, or small compared to the probability that a target will move substantially out of alignment between imaging samples. It is also anticipated that other aspect of the patient's state other than the target itself can be tracked, such as OARs, heart rate, etc.
Real-time imaging modalities may use the same underlying imaging technology as IGRT modalities with implementation differences to increase speed, or they may use different imaging technologies altogether. For example, the kV imager used to generate CBCT images in real time can be used during the treatment itself, but only to give projectional information; the missing information must be deduced using an intelligent algorithm. MRI imagers can be configured to 1D navigators, 2D planes, or coarser 3D images, to increase imaging speed. Ultrasound imaging and RF beacons can be directly used to track the target in real time. Surface markers, surface cameras, ECG, EEG can give partial information which can be used to help estimate the target position.
In many cases, it is useful to use different modalities for IGRT target tracking, in some cases using the same underlying imaging technology, and in other cases, different technologies. IGRT has the luxury of not being necessarily real-time, and so the time budget can be used to generate richer 3D information, which is not acceptable for target tracking. IGRT typically operates under the assumption that the target is sufficiently static. However, practically the structure of interest might move significantly during the IGRT acquisition. In that case, the positional information obtained during the IGRT stage can no longer be accurately used by the target tracking stage. For example, a prostate patient may cough move, pass gas, or have significant bladder filling such that the position of his prostate may change during the transition between IGRT and target tracking modalities.
Therefore, there is a need for implementing different modalities for IGRT and target tracking that does not assume that no motion occurs during IGRT and during the transition between modalities.